The present invention relates to apparatuses and methods for determining a coincidence or matching of a position with a reference position, as may for example be used for locating or navigating mobile terminals in a wireless communication network, in particular.
For finding persons with mobile terminals, various localization technologies are available. The probably best-known system for localization and/or navigation in the outdoor area is the satellite-aided global positioning system (GPS). For the localization and/or navigation inside of buildings and/or in an indoor area, various approaches are known, such as infrared systems, RFID (radio frequency identification) systems or also field strength evaluations of IEEE 802.11 WLAN (wireless local area network) networks. At the moment, the GPS system is available in reliable manner only for the outdoor area. More recent extensions, such as highly sensitive receivers or the so-called A-GPS (assisted GPS) represent attempts of making the technology usable also within buildings. Here, A-GPS combines the use of the satellite-based GPS system with reception of so-called assistance information from cellular mobile radio networks. Presently, however, these technologies do not yet have the desired mean accuracies. Infrared systems and RFID systems generally are not available with complete coverage and are bound to specific prerequisites.
Due to the increasing dissemination of wireless radio networks, for example based on the WLAN standard, these wireless networks lend themselves as a basis for new localization methods.
Common, previously used localization methods are based on triangulation, neighborhood relations, lateration by means of time measurement of lateration by means of field strength evaluation, for example. These methods are localization methods in which either a position of stationary transmitters and/or base stations has to be known, or in which training has to take place beforehand at typical positions in an environment to be covered by the localization method.
In WLAN-based localization systems, so-called received signal strength (RSS) fingerprinting often is employed as a basic method. This method is based on the assumption that signal strengths of radio signals of several radio stations received and/or receivable at a current location uniquely characterize the location or position. If there exists a database containing, for a number of reference locations or reference positions, identifications of radio stations received and/or receivable there, as well as the field strengths of corresponding radio signals, the current position may be inferred from a set of current measurement values (transmitter identifications and signal strength values) by performing matching between currently measured measurement values and the reference values of the database. For each reference point, this matching assesses how similar its previously recorded measurement values and/or reference values are to the current measurement values of the current position. The most similar reference point(s) then determine an estimate for the current location.
For a reference database, the signal strength is determined experimentally at a sufficient number of points by test measurements. Thereby, a database containing a list of base stations (access points) with the respectively associated reception field strength and quality for each position at which a test measurement was performed is created. In a WLAN implementation, such a reference database may, for example, include the following parameters:
RIDMACRSSIPGSXYZMAPNRCREATED100.0D.54.9E.17.8146530100579515627150012.03.07 12:42100.0D.54.9E.1A.BA6726090579515627150012.03.07 12:42100.0D.54.9E.1D.647200288579515627150012.03.07 12:42100.0E.6A.D3.B9.8B59531100579515627150012.03.07 12:42100.0F.A3.10.07.6C4646496579515627150012.03.07 12:42100.0F.A3.10.07.FB7448894579515627150012.03.07 12:42100.0F.A3.10.09.SF7237597579515627150012.03.07 12:42200.0D.54.9E.17.81541381001439915451150012.03.07 12:43200.0D.54.9E.18.1D76560111439915451150012.03.07 12:43200.0D.54.9E.1A.BA62318941439915451150012.03.07 12:43200.0D.54.9E.1D.6471348961439915451150012.03.07 12:43200.0E.6A.D3.B9.8B453931001439915451150012.03.07 12:43200.0F.A3.10.07.6C66853961439915451150012.03.07 12:43200.0F.A3.10.07.FB722511001439915451150012.03.07 12:43200.0F.A3.10.09.5F70990901439915451150012.03.07 12:43300.0D.54.9E.17.81582911002458315627150012.03.07 12:43300.0D.54.9E.18.1D78610682458315627150012.03.07 12:43300.0D.54.9E.1A.BA62153982458315627150012.03.07 12:43300.0D.54.9E.1D.6464187902458315627150012.03.07 12:43300.0E.6A.D3.B9.8B328511002458315627150012.03.07 12:43300.0F.A3.10.07.6C69006962458315627150012.03.07 12:43300.0F.A3.10.07.FB71749922458315627150012.03.07 12:43300.0F.A3.10.09.5F71482832458315627150012.03.07 12:43300.0F.A3.10.09.8071000402458315627150012.03.07 12:43Thus, the table includes the following information:                the reference point identification (RID)        the MAC addresses of the received stations        the reception field strengths of the access points (RSSI; 46560 means −46.560 dBm)        the position in Cartesian, metric coordinates (x, y, z; 24583 means 245.83 m), as well as        the time of measurement value capture.        
WLAN signals, which theoretically are to be measured at relatively low field strength only, exhibit a relatively unreliable behavior with respect to “measurable” or “not measurable”. The column of PGS (“percentage seen”) indicates, in percent, how often this station was seen during measurement value capture (i.e. PGS=90 means that the station was measured in 9 out of 10 measurements, on average). The PGS value is determined when training the reference positions and/or the reference measurement packets for each radio transmitter and is to be understood as a measure for its reliability. Within a certain measurement time window, there is a defined number of possible measurement values from a radio transmitter by way of a fixed sampling interval of e.g. 200 ms. The PGS value is a percentage value of the (RSSI) values of a radio transmitter really measured within the measurement time window in relation to the potentially possible ones. Reference points are captured over a longer time window (e.g. 6 to 10 s) e.g. every 200 ms in the ideal case during calibration. In this respect, FIG. 5 shows an exemplary waveform of a reception signal of a certain radio transmitter, which can be received only relatively unreliably at the measurement position. Over a measurement time window of 10 s, the certain radio transmitter is receivable only for about 3 s, which yields a PGS value of about 30% for this radio transmitter.
For the localization, currently captured measurement values are compared with the database. The most similar one or an integration of the most similar reference values is accepted as current position. Several methods are possible for the matching; the most widely used one is least distance in signal space.
RSS fingerprinting provides good results in the indoor and outdoor areas. Owing to the fact that the setup locations of the stationary radio transmitters do not have to be known, the method also is well-suited for unknown environments with unknown infrastructure. Fingerprinting approaches for the matching assume a fixed, invariable infrastructure. Many known solutions further assume a limited area in which signals of each radio transmitter can be received everywhere.
So as to determine a coincidence or matching of a current position with a reference position, RSSI values of a number of radio transmitters, previously recorded transmitter identifications of which at the reference position are identical with transmitter identifications provided at the position, often are compared with each other in conventional methods. The smaller the deviation of the RSSI values between the radio transmitters with identical transmitter identification, the higher the coincidence of the current position with the reference position. However, this procedure also poses the risk of faulty position estimation—for example when the number of the radio transmitters, previously recorded transmitter identifications of which at the reference position are identical with transmitter identifications provided at the position, is small, and thereby also a small RSSI value deviation is determined, which may lead to a coincidence falsely estimated to be good.